# 按照派生類型打印對象 ## 例子 #include <iostream> using namespace std; class Shape { public: virtual void draw () {} }; class Circle : public Shape { int radius; public: Circle () { radius = 1; } void draw () { cout << "drawing a circle...\n"; } }; class Square : public Shape { int height; public: Square () { height = 2; } void draw () { cout << "drawing a square...\n"; } }; void drawShape (class Shape &p) { p.draw (); } int main (void) { Circle a; Square b; drawShape (a); drawShape (b); return 0; } ## 技巧 在gdb中，當打印一個對象時，缺省是按照聲明的類型進行打印： (gdb) frame #0 drawShape (p=...) at object.cxx:25 25 p.draw (); (gdb) p p $1 = (Shape &) @0x7fffffffde90: {_vptr.Shape = 0x400a80 <vtable for Circle+16>} 在這個例子中，p雖然聲明為class Shape，但它實際的派生類型可能為class Circle和Square。如果要缺省按照派生類型進行打印，則可以通過如下命令進行設置： (gdb) set print object on (gdb) p p $2 = (Circle &) @0x7fffffffde90: {<Shape> = {_vptr.Shape = 0x400a80 <vtable for Circle+16>}, radius = 1} 當打印對象類型信息時，該設置也會起作用： (gdb) whatis p type = Shape & (gdb) ptype p type = class Shape { public: virtual void draw(void); } & (gdb) set print object on (gdb) whatis p type = /* real type = Circle & */ Shape & (gdb) ptype p type = /* real type = Circle & */ class Shape { public: virtual void draw(void); } & 詳情參見[gdb手冊](https://sourceware.org/gdb/onlinedocs/gdb/Print-Settings.html#index-set-print) ## 貢獻者 xmj xanpeng